The present invention relates generally to nickel rechargeable cells, such as nickel metal hydride (NiMH) cells, and more specifically to a method and apparatus for automatically reversibly terminating a cell charging process. This invention may also be employed in nickel cadmium (NiCd) cells.
For greater convenience and portability, many modern electrical appliances and consumer products may be operated to draw electric current from batteries of standard size and electrical performance. For convenience and economy, various rechargeable batteries have been developed, such as nickel metal hydride cells and the like.
Metal hydride cell technology provides excellent high-rate performance at reasonable cost when compared to nickel cadmium and lithium ion technology. Moreover, metal hydride cells have about a 50% higher volumetric energy density than NiCd cells and about equal to lithium ion cells. The internal chemistry of metal hydride rechargeable cells has an impact on the ability to charge such cells. Issues affecting the ability to charge nickel rechargeable cells arise as a result of the internal chemistry of such cells. When a nickel rechargeable cell approaches a full charge state, oxygen is generated at the cathode as follows:4OH−→O2(gas)+2H2O+4e−
The oxygen gas diffuses across a gas-permeable separator to the anode where it is recombined into cadmium hydroxide or water as follows:½O2(gas)+H2O+Cd→Cd(OH)2+Heat @ Cadmium anode½O2(gas)+H2→H2O+Heat @ Hydride anode
When recharging such cells, it is important to ascertain when the cell has become fully charged. For example, if a cell were to become overcharged for an extended period of time, the pressure buildup within the cell could cause the cell to fail as well as electrolyte to leak, thereby further subjecting the charger to potential damage.
Metal hydride rechargeable cells are typically recharged by applying a constant current rather than constant voltage to the cells. In this scheme, cell voltage increases gradually until the cell approaches full charge whereupon the cell voltage peaks. As the cells reach the overcharge state, the released heat causes the cell temperature to increase dramatically, which in turn causes the cell voltage to decrease. Cell pressure also rises dramatically during overcharge as oxygen gas is generated in quantities larger than the cell can recombine. Unfortunately, it is known that the rate of pressure change is several orders of magnitude faster than the rate of voltage or temperature change. Thus, conventional constant current charge interruption methods cannot support a very fast charge rate without risking internal pressure buildup, rupture, and electrolyte leakage. For this reason, metal hydride cells may be provided with safety vents.
One common way to reduce pressure buildup at the full-charge state is to provide an anode having a excess capacity of greater by 40-50% more than the cathode, a gas-permeable separator, and limited electrolyte to accommodate effective diffusion of gasses. This avoids the production of hydrogen gas at the anode while permitting the oxygen to recombine with the anode material. When a cell reaches full charge, oxygen gas continues to be produced at the cathode, but hydrogen is not produced from the anode. If hydrogen were produced, the cell could rupture from excess pressure. The oxygen recombination reaction therefore controls the cell pressure, as is illustrated in FIG. 1. The oxygen gas then crosses the separator and reacts with the anode material. Downsides of this arrangement include reduced cell capacity and corresponding shorter cell cycle life due to degradation of the anode from overcharge with oxidation and heat.
It is important to stop charging a cell or plurality of cells when a full charge state is reached to avoid possible cell rupture or leakage due to the increasing internal gas pressure. Conventional metal hydride rechargeable cells cannot themselves signal a suitable charge termination point. One must instead rely upon expensive and sophisticated detection circuitry in an associated charger device to determine when charging should end. Charge termination is typically determined by the detection circuitry based on (1) peak cell voltage, (2) peak cell temperature (TCO), (3) duration of charging time, (4) −dV, and (5) dT/dt. Each known method for terminating a constant current charge has disadvantages. For example, time-based termination can be unreliable except at very low charge rates because the cell can become overcharged before termination.
Charge termination based on peak voltage can be unreliable at the end of the charging period because an over-voltage condition can exist before termination. Termination based on a voltage decline (−dV) is necessarily associated with oxygen recombination and the accompanying detrimental temperature rise. In practice, this means that voltage detection must be accurate and fast. Unless the ambient temperature is steady, it can be difficult to accurately measure a change in voltage. Moreover, when the charge rate is slower than 0.3 C, the voltage drop measurement is too small to be detected accurately. A charge rate of 1 C draws a current equal to the rated capacity of the electrochemical cell or battery. Termination based only on peak temperature is also easily affected by ambient temperature changes.
Termination based upon the rate of change in temperature over time (dT/dt) is somewhat more reliable than detecting an absolute temperature change because it is less subject to effects caused by ambient temperature change and because there is less negative effect on cycle life, but it is still based on heat which is detrimental to cell performance and cycle life. This is because temperature increases faster, and, in fact, precedes, the drop in voltage. Accordingly, there is somewhat less risk of rupture and leakage than in the other methods noted above. This makes it the most common charge termination method in use today.
Others in the art have sought pressure based mechanisms for breaking the connection between the electrode and the cell terminal when pressure exceeds a predetermined level. For example, U.S. Pat. No. 5,026,615 discloses a pressure-sensitive switch in an end cap assembly that comprises a conductive spring member, a nonconductive fulcrum member and a moveable conductive member. The conductive spring member is in electrical connection with a terminal on one end and with the moveable conductive member on the other end. The moveable conductive member is in turn in electrical connection with an electrode. As the internal cell pressure increases, the moveable conductive member exerts force on the spring member, which pivots on the nonconductive fulcrum member and disconnects from the terminal. This patent therefore requires a first and second contact, one of which being movable with respect to the other and rotatable about a fulcrum in order to pivot with respect to the other contact. This arrangement requires more essential parts than necessary, and further requires that the assembly be constructed with tight tolerances, thereby increasing complexity as well as the cost of production.
Other examples of these technologies include U.S. Pat. Nos. 5,747,187, 5,405,715, 5,741,606, 5,609,972, 6,018,286, 6,078,244, and 6,069,551, all of which are incorporated herein by reference as if set forth in their entirety. Some such mechanisms prevent a pressure-induced rupture of the cell but in doing so permanently disable the cell. In other cases, reversible switch devices prevent cell rupture, but do not detect an early charge termination state to avoid heat build up and to ensure superior cell performance and cycle life.
With constant voltage charge, on the other hand, the charging current is high at the beginning of the charge, when the cell can accept higher currents, and then decreases to lower levels as the cell approaches full charge. When constant voltage charging, the above-noted signals for the end of a constant current charge process are not useful because as the cell approaches the full charge state, the cell voltage is constant and the cell temperature is leveling. Like a constant current charge approach, charging time cannot be used for the constant voltage charge when the charge rate is higher than 0.3 C due to run away of pressure that can damage devices. As a result of these shortcomings it has been difficult to identify an effective termination signaling means and constant voltage charging for metal hydroxide cells has therefore been generally considered to be impractical.
With alternating current charge, the charging current may be modulated at a defined frequency or combination of frequencies to produce a net positive current that enables the cell to become charged. An alternating current charge can provide a fast charge with less pressure buildup and lower temperature increase than constant current or constant voltage charge. However, when using an alternating current charge, the above-noted signals for the end of a constant current charge process are not useful because as the cell approaches the full charge state, changes in the cell voltage are difficult to detect above the voltage response to the applied alternating current. As a result it has been difficult to identify an effective termination signaling means and alternating current charging for metal hydroxide cells has also therefore been generally considered to be impractical. It should be appreciated that an alternating current charge is used throughout the present disclosure to mean a varying current that produces a net positive charge, such as a modulated alternating current. For example, an alternating current may be half-wave rectified or full-wave rectified to produce a series of current pulses, or an alternating current may be offset by a desired DC current.
Published Australian patent application number 199926971 A1 discloses a method for fast charging a nickel metal hydride battery in an implant by transcutaneous transmission of electric power from an external power-transmission part to a power-receiving part in the implant. The patent application considers the desirability of an initial rapid high-current charge phase when the internal cell resistance is low, followed by a second lower-current, constant cell voltage charge phase to ensure that the cell is charged only with as much energy as the electrochemical state allows, without excess gassing or heating of the cell. Harmful effects on the battery are precluded while, at the same time, the charging rate remains high. In the method disclosed therein, a first of two charging phases includes the step of allowing a relatively high constant charging current to flow to the power receiving part while the cell voltage rises until it reaches a predetermined limiting charging voltage. In the second charging phase, the charging current is lower than the current level at the end of the first phase while the cell voltage is kept at least approximately at the predetermined constant voltage value. In the Australian patent application, the second charge phase ends when an associated micro-electronic controller determines that the rate of change of the charging current over time does not reach a predetermined slope. This cumbersome two-step constant current/constant voltage approach is typical of prior approaches in the art.
In summary, as the metal hydride rechargeable cell reaches its fully charged state, oxygen is evolved from the cathode, thereby increasing the internal cell pressure and driving the exothermic oxygen recombination reaction. At a very high constant current charge rate, usually less than one hour, charge current is still very high at the end of charge. This results in severe heating of the cell and shortened cycle life. The available methods of terminating constant current charge are not very reliable when cell temperature is high. In addition, cell heating is detrimental and it is desirable to terminate the charge before significant cell heating at the stage where damaging pressure begins to rise within the cell.
What is therefore needed is a method and apparatus for more accurately determining the charge termination point for a cell that is fully rechargeable under constant voltage, constant current, and alternating current/voltage charging.
What would be desirable is a reversible regulating switch that is responsive to a stimulus for determining a charge termination point that is less complex and less destructive than those currently available.
What is also desirable is a more cost-efficient and reliable charge termination detection apparatus than that currently achieved, and that is compatible with conventional rechargeable batteries.